The present invention relates to a device for detecting a physical variable for a process fluid via differential measurement.
It is known that, in systems for controlling industrial processes, for the purpose of detecting/measuring one or more physical variables of a process fluid, for example, absolute, relative or differential pressure, flow rate, level, and the like, purposely designed field devices are widely used.
In one of the most widespread embodiments, said field devices comprise an appropriately configured casing, housed inside which are the various components designed for detection, amongst which typically a pressure sensor (hence the term “pressure transmitters”). The use of a pressure sensor as transduction element in fact enables convenient derivation, by one or more relative, differential or absolute pressure measurements, of measurement values for other physical variables of the process fluid that are being controlled, for example the level, which would otherwise be more difficult to transduce directly. In the casing of the transmitter there are then provided other components, amongst which: primary electronic circuits, which are designed to process the signals coming from the pressure sensor; secondary electronic circuits designed for processing the signals coming from the primary electronic circuits and hence responsible for managing the communication with other transmitters or with the control unit; display units for displaying in loco the variables detected; a terminal board for the various connections; as well as electrical circuits for supply of the components, etc.
To carry out the operations of detection and measurement required, each pressure transmitter is provided with one or more pressure-sensitive elements that interface with the process and are usually designated by the terms of “separator assemblies”, or “diaphragms” or “separation membranes”, or other equivalent terms.
In particular, differential pressure transmitters are provided with two separator assemblies, each of which comprises a purposely provided elastic element, typically a flexible metal membrane that is fixed on a support, which is also usually made of metal. The separator assembly is positioned on the instrument so that the membrane will have an outer wall exposed to the process fluid and an inner wall coupled hydraulically to the pressure sensor, usually via an incompressible fluid, such as, for example, a silicone oil, contained in a purposely provided capillary connection tube. In this way, the pressure exerted by the fluid on each of the flexible membranes causes deformation thereof and is transferred via the hydraulic-coupling fluid to the pressure sensor. In turn, the sensor “senses” the pressure on the two branches of the instrument and transmits the corresponding signals to the electronics for the various processing operations.
In some conditions of application, such as, for example, for measuring the level of a fluid contained within a tank, it is necessary to carry out the measurements in a position that is remote with respect to the instrument and/or in two different positions of the process, which may even be at considerable distances apart from one another and from the body of the transmitter itself. In this case, the differential pressure transmitter is equipped with purposely designed remote separators. Said remote separators also comprise similar pressure-sensitive elements, typically an elastic membrane appropriately fixed on a supporting separator body. A purposely provided capillary tube, filled with transduction or transmission fluid, preferably an incompressible silicone oil, sets the elastic membrane of the remote separator in direct hydraulic communication with the elastic membrane housed directly on the main body of the instrument.
In said applications, any detection made may be subject to errors determined by differences of temperature of the process in which the transmitter is inserted, as well as by differences of ambient temperatures, and in particular differences of temperature that can be encountered throughout the structure of the two measurement branches constituted by the main body of the instrument, the two, remote separators, and the surrounding parts.
Consequently, the main purpose of the present invention is to provide a device for detecting a physical variable for a process fluid that will enable a sensible reduction of the errors of measurement caused by differences of process and ambient temperatures between the two branches of the instrumentation for detection and the surrounding parts.
This purpose is achieved by a device for detecting a physical variable for a process fluid according to what is specified in the claims.